1. Field of the Invention
The present invention relates to a belt conveying apparatus in which a belt entrained about a plurality of rollers is rotated by receiving a rotational driving force imparted by at least one driving roller, and to an image forming apparatus in which a photosensitive material and an image receiving material are superposed on each other and heat developed in a heat developing processing section of the apparatus.
2. Description of the Related Art
An image forming apparatus is proposed which records an image on a photosensitive material, carries out thermal developing processing in a state in which the photosensitive material having an image recorded thereon and an image receiving material are superimposed, and transfers the image to an image receiving material.
In the thermal developing processing, the two sheet materials, which are formed by laminating the photosensitive material and the image receiving material in the presence of an image forming solvent (for example, water), are wound about the circumferential surface of a heating drum, and are then conveyed while being held by a belt. The heating drum is provided with a halogen lamp, as a heating source, in a shaft core thereof. Heat generated when the halogen lamp is lit heats the circumferential surface of the heating drum. The photosensitive material and the image receiving material undergoes thermal development transfer processing due to this heat, so that an image on the photosensitive material is transferred to the image receiving material.
The belt is entrained about a plurality of rollers. One roller among the plurality of rollers is a driving roller. The belt circles about the plurality of rollers in synchronism with rotational speed of the heating drum, due to rotational driving force of the driving roller. The problem of the belt moving sideways as it is conveyed about the plurality of rollers cannot be ignored. The image forming apparatus is for originals of a comparatively large size (for example, 841 mm.times.1189 mm, or 728 mm.times.1030 mm) and width of the belt has become larger in accordance with the size of the original. The amount the belt moves sideways during conveyance increases as the transverse dimension of the belt increases. This sideways movement is caused by the flatness of the circumferential surface of the rollers about which the belt is entrained (the rollers have the same radius throughout in the axial direction).
As a result, it has been proposed to make each of the driven rollers, which are the rollers other than the driving roller, in a trapezoidal form, by tapering a portion in a vicinity of both ends in the axial direction thereof (for an example, refer to Japanese Patent Application Laid-Open (JP-A) No. 2-66036). Due to this structure, the belt that moves sideways onto the tapered portions increases in speed, since the tapered portions have an increased linear speed due to an increased diameter. As a result, the belt returns to its original position. Accordingly, sideways movement is prevented from worsening.
However, with the above-described prior art, since the tapered portions and the driven roller that is the base body for the tapered portions rotate integrally, the force apt to move the belt sideways and the force apt to reverse the sideways movement may balance each other. As a result, even if sustained sideways movement is not caused, the belt moves in the transverse direction at certain positions, which causes the photosensitive material and the image receiving material to become out of registration. At this time, uneven transfer occurs, which reduces image quality.
In the prior art, there is disclosed a technology wherein, in cases where sideways movement occurs, positions of both ends of some of the driven rollers are independently adjusted in the axial direction. Namely, by inclining the axial line of the driven roller in the direction opposite to the direction of the sideways movement, the sideways movement can be reversed.
However, since the direction of inclination is also inclined such that the axial line is inclined in the direction the belt is conveyed, excessive torsion takes place and the belt may form slack. Due to this slack, force holding the photosensitive material and the image receiving material varies across the transverse direction thereof, and uneven transfer may occur as a result.